Screen printer with ink level sensing and control

ABSTRACT

An automated printing ink supply device including an ink storage container, a slider valve through which the flow of printing ink onto an ink receiving area can be controlled and a mechanism including a liquid level detector for operating the slider valve. By this menas, the ink can be automatically supplied to the ink receiving area on the screen during the time an endless belt of the automatic screen printing machine is stopped to compensate for the amount of the printing ink that has been lost or deposited on the material to be printed.

United States Patent [1 1 Tatebe Dec. 18, 1973 SCREEN PRINTER WITH INK LEVEL SENSING AND CONTROL [75] Inventor:

[22] Filed: Sept. 20, 1971 [21] Appl. No.: 181,732

Masaharu Tatebe, Kyoto, Japan [30] Foreign Application Priority Data 3,535,629 10/1970 Gibson et a1 222/64 X 3,604,594 9/1971 lsreeli et a1. 222/64 3,170,479 2/1965 Mueller 222/64 2,242,182 5/1941 McCann 101/115 UX 3,252,411 5/1966 Black l0l/l24 3,256,809 6/1966 Gsell 101/123 3,263,603 8/1966 Fuchs l0l/l23 Primary ExaminerRobert E. Pulfrey Assistant ExaminerR. E. Suter Att0rneyE. F. Wenderoth et al.

[57] ABSTRACT An automated printing ink supply device including an ink storage container, a slider valve through which the flow of printing ink onto an ink receiving area can be controlled and a mechanism including a liquid level detector for operating the slider valve. By this menas, the ink can be automatically supplied to the ink receiving area on the screen during the time an endless belt of the automatic screen printing machine is stopped to compensate for the amount of the printing ink that has been lost or deposited on the material to be printed.

6 Claims, 22 Drawing Figures Sept. 28, 1970 Japan ..45/85249 [52] U.S.Cl ..101/123,101/115,101/364 [51] Int. Cl B41f 15/40 [58] Field of Search lOl/1l4,ll5, 123, 101/124, 129, 364, 119, 126; 222/56, 64, 67

[56] References Cited UNITED STATES PATENTS 1,815,021 7/1931 Bland 101/123 2,206,570 7/1940 Johnston 101/115 3,419,413 12/1968 Fuforbia 101/123 UX 2,529,250 11/1950 Fordyce 101/123 2,955,567 10/1960 Way et a1 101/115 X 3,631,800 l/1972 Mignone et a1 222/64 X PATENTED DEC 18 I975 SHEEI 010F 15 PATENTEU DEC 18 I975 SHEET 03 0F 15 I I4 E I5 290 270 PATENTEDUEC18 I975 3,779,161

sum user 15 I NH, 15 i W 5 an 480 2 no PATENTED DEC 1 8 I975 SHEET OBUF 15 Fig. 7

30h I 6 30 2 Oc(30|) 3O 7 Fig. /0

K13 Power Sou rge a w w SC PATENTEI] DEC 18 ms SHEET 08 0F 15 Qmm QNN

PATENTEDUEC18 I975 SHEET DSUF 15 PATENTED on: 1 8 ma 3.779.161

sum 11 0F 15 PATENTED DEB 1 8 I975 SHEET 120F15 PATENTEDBEB 18 ms sum 1n 0F 15 Fig. 2/

SCREEN PRINTER WITH INK LEVEL SENSING AND CONTROL The present invention relates to an automatic screen printing machine and, more particularly, to an improvement in an automatic screen printing machine wherein an automated printing ink supply device is provided for automatically supplying a proper amount of printing ink onto the screen to compensate for the amount of said ink that has been deposited on material to be printed.

Specifically, the present invention pertains to the automated printing ink supply device so designed as to maintain at a predetermined value the position of a liquid surface of the printing ink contained in an ink receiving area on the screen throughout the screen printing operation.

The automatic screen printing machine to which the present invention is applicable, in general, includes an endless belt made of hard or soft rubber material and adapted to carry a textile material to be printed which material is adhered to the upper flat surface of the forwardly moving, relative to the direction of movement of the material through the machine, run of said belt, said endless belt being intermittently moved so that, during the time it is standing still, a screen frame (or screen frames in the case of multi-colour printing) having a screen is downwardly shifted together with a pair of squeegees until the screen substantially contacts the textile material on the endless belt and said squeegees are subsequently traversed on the screen in the direction of the width of the belt and the machine proper to deposit a printing ink, which has been contained on the screen, onto the textile material through the screen to complete the ink depositing operation.

After the completion of the ink depositing operation at which time the pair of squeegees is normally transferred from the initial position to another position, the screen frame is upwardly shifted together with the squeegees and the endless belt is again moved, thus completing a cycle of squeegee movement.

Despite the fact that the printing machine carries out consecutive cycles of the ink depositing operation in an automatic manner, a primitive method has usually been employed to supply printing ink onto a printing ink receiving area on the screen in which an attendant worker utilizes a ladle-like instrument for supplying the printing ink from an ink storage container placed at one side of the machine proper. In multi-color screen printing, a storage container is naturally required for the screen frame for each color so that a number of attendant workers corresponding to the number of colors is usually required. If a single attendant worker is forced to take charge of supplying the printing ink of different color to, for example, three or four screen frames, he will be confused as to where to supply the printing ink of what color and, in an extreme case, will neglect to supply the printing ink of a specific color to one of the screen frames since each printing operation is carried out in a matter of seconds. If this event occurs, the textile material will have an unevenly colorprinted appearance or will otherwise be spoiled.

As is well known to those skilled in the art, the printing ink usually employed has a relatively high viscosity ranging from about 4,000 to 90,000 c.p.s. and, accordingly, the weight of the printing ink imposed on a unit area of the screen, namely, the amount of the printing ink normally contained in an ink receiving area on the screen, is one of the major factors that affect the success of the printing performance. For this reason, it is desirable to maintain at a constant value, the amount of the printing ink contained in the ink receiving area on the screen. However, this cannot be achieved by the above mentioned primitive method.

To eliminate the disadvantages inherent in the above mentioned primitive method of supplying the printing ink, there has been proposed an arrangement wherein a combination of a pumping device with a choke valve is provided for the this purpose. In this instance, the choke valve is normally positioned above the ink receiving area on the screen and the printing ink is continuously supplied through the choke valve onto the screen by the pumping device at a relatively low rate.

However, even this proposed arrangement has a disadvantage in that, in view of the fact that the ink receiving area on the screen is defined by the squeegees in cooperation with the screen frame, the printing ink contained therein begins to spread over the surface of the screen following the moving squeegee as the latter is traversed to another position away from the position where the choke valve is located and an excessive amount of the printing ink is, therefore, deposited on the textile material to be printed during each period the endless belt is stopped.

Accordingly, an essential object of the present invention is to provide an automated printing ink supply device for use in screen printing machines which is capable of supplying a proper amount of printing ink onto the ink receiving area on the screen to compensate for the amount of said ink that has been lost or deposited on the textile material to be printed.

Another object of the present invention is to provide an automated printing ink supply device for use in screen printing machines which includes a slider valve having a passage one end of which communicates with an ink storage container and the other end of which is directed to the ink receiving area, said slider valve being so designed as to permit the flow of printing ink onto the ink receiving area without any formation of droplets of ink about said other end which may unnecessarily waste the ink and/or contaminate portions of the machine.

A further object of the present invention is to provide an automated printing ink supply device for use in screen printing machines wherein a slider valve is provided which is operable by the logical product of electrical signals, one of which is fed from a liquid level detector and which is representative of a deviation of the position of the liquid surface of the printing ink contained in the ink receiving area relative to a predetermined position and the other of which is fed from a limit switch or electromagnetic device and which is representative of the return of the squeegees from a position remote from the ink supply device, whereby a proper amount of printing ink can be supplied onto the ink receiving area only when the liquid surface position of the printing ink is lowered beyond the predetermined position while the squeegees is completely returned to the initial position adjacent to the ink supply device.

A still further object of the present invention is to provide an automated printing ink supply device for use in screen printing machines wherein means is provided for operating the slider valve to permit the flow of the printing ink from the ink storage container onto the ink receiving area to maintain at a predetermined value the amount of the printing ink contained in the ink receiving area on the screen.

A still further object of the present invention is to provide an automated printing ink supply device for use in screen printing machines wherein a printing ink storage container is movably supported whereby transportation of the ink storage container from the printing position to another or a preparation room where the printing ink is prepared can be facilitated.

A still further object of the present invention is to provide an automated printing ink supply device for use in screenprinting machines which contributes to the reduction of the number of attendant workers in charge of ink supply stations for each screen printing machine without causing any situation such as that in which an attendant worker has heretofore been oftentimes confused as to where to supply printing ink of what color.

A still further object of the present invention is to provide an automated printing ink supply device for use in screen printing machines which contributes to the reduction of labour cost and is effective to produce printed textile material at low cost as compared with the labour cost that has been incurred by the employment of a number of attendant workers.

These and other objects and features of the present invention will become apparent from the following full description of the present invention taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, in which;

FIG. 1 is a schematic side view of an exemplary type of screen printing machine arrangement to which the present invention is applicable,

FIG. 2 is a schematic diagram showing a carriage structure for movably supporting the printing ink storage container,

FIG. 3 is a side view, partly in section of FIG. 2,

FIG. 4 is a top plan view of FIG. 2,

FIG. 5 is a schematic diagram showing a slider valve employed in the automated printing ink supply device of the present invention and its associated parts,

FIG. 6 is a side sectional view of FIG. 5,

FIG. 7 is a bottom plan view of FIG. 5,

FIG. 8 is a schematic perspective view of the slider valve showing the details thereof on an enlarged scale,

FIG. 9 is a side sectional view of FIG. 8,

FIG. 10 is an electrical wiring diagram which may be employed for operating the printing ink supply device of the present invention,

FIG. 11 is a schematic perspective view of a modification of the'embodiment shown in FIGS. 5 to 7, in which a liquid level detector in the form of a float is employed,

FIG. 12 is a view similar to FIG. 5, but showing a further embodiment of the present invention in which the slider valve is adapted to be operated by an electromechanical arrangement,

FIG. 13 is a side view of FIG. 12,

FIG. 14 is a bottom plan view of FIG. 12,

FIG. 15 is a schematic perspective view of a portion of FIG. 12,

FIG. 16 is a schematic diagram showing a still further embodiment of the present invention wherein the ink storage container is placed on the floor while a combination of a pumping device and a choke valve connected with the slider valve is employed,

FIG. 17 is a top plan view of FIG. 16,

FIG. 18 is a schematic sectional view of the ink supply device in a still further embodiment of the present invention,

FIG. 19 is a view similar to FIG. 1, but showing an over-head carriage arrangement for suspending each ink storage container from the ceiling of a printing shop,

FIG. 20 is a side view of FIG. 19,

FIG. 21 is a top plan view of FIG. 19, and

FIG. 22 is a schematic diagram showing the arrangement of rails in the printing shop for the over-head carriage arrangement of FIG. 19.

Before the description of the present invention proceeds in connection with preferred embodiments thereof with reference to the accompanying drawings, it is noted that like parts are designated by like reference numerals throughout the accompanying drawings.

Referring now to FIG. I, the automatic screen printing machine generally comprises a supply unit I, a printing unit II and a take-up and drying unit III. This automatic screen printing machine further includes an endless belt A adapted to be circulated round a pair of belt drums l and 2, one of which is accommodated in the supply unit I and the other of which is accommodated in the printing unit II at a position remote from the supply unit. As shown, this endless belt is deflected substantially at right angles by a pair of deflection rollers 3 and 4 from the vertical run to the horizontal run. The belt A is adapted to be driven by the roller 3 or another roller (not shown) disposed at a suitable position,

which is in turn drivingly connected with a suitable motor. The drive roller 3 has a counter pressure roller 5, around which material to be printed, for example, cloth B, is fed from a roll C of cloth by means of a deflection roller 6. The cloth B passes through the contact point between the drive roller 3 and the counter pressure roller 5 and is usually, as known to those skilled in the art, adhered to the adjacent surface of the endless belt A on which an adhesive agent has been spread by means of a gluing roller 7 positioned below the drive roller 3 and behind the counter pressure roller 5 with respect to the feed direction of the cloth B.

The drive roller 3 may be driven either continuously or intermittently.' However, the printing operation requires that the endless belt A stand still relative to ink depositing means for a short time, during which a single screen frame or a plurality of screen frames D are moved downwardly to enable an ink depositing operation to be performed. Accordingly, in the case where the ink depositing operation is to be performed while the drive roller 3 is continuously driven such as shown in FIG. I, both drums 1 and 2 are adapted to move together in one direction. In other words, while the drive roller 3 is continuously driven, the drum 1 moves from a downwardly shifted position as indicated by the dotted line to an upwardly shifted position to take up the length of the endless belt A fed by the drive roller 3 toward the drum 1 and, at the same time, the drum 2 moves toward the supply unit I the same distance as the drum 1 is moved, so that the portion of the belt A situated in the printing unit II will be at the stand-still position relative to the screen frames D.

In any event, during the stand-still of the endless belt A, each screen frame D is moved from an upwardly shifted position to a downwardly shifted position in which the cloth B on the belt A is closely covered with a screen carried by the screen frame D. At this time, a pair of spaced parallel squeegees is moved over the screen in the widthwise direction of the cloth B to deposit a printing ink on the flat surface of the cloth B through the screen and, thereafter, the screen frame D is moved to the upwardly shifted position together with the pair of the squeegees to complete the ink depositing operation.

The cloth B that has been printed during its travel through the printing unit II is separated from the belt A at the end of the printing unit II and is fed into the take-up and drying unit III in which the printed cloth is dried and rolled up.

The structure of the automatic screen printing machine so far described may be any conventional type such as that manufactured by Fritz Buser AG of Switzerland and, accordingly, the details thereof are herein omitted for the sake of brevity. More specifically, the present invention pertains exclusively to the printing unit ll of the automatic screen printing machine and is directed to the automation of a printing ink supply system. A portion of said printing unit II to which the present invention is applicable is illustrated particularly in FIGS. 2 through 4 on an enlarged scale.

Referring now to FIGS. 2 through 4, the automated printing ink supply device according to the invention generally includes a bucket-like ink container E in which a proper amount of printing ink is contained, a solenoid operated valve assembly F for adjustably controlling the flow of printing ink from the container E onto a screen (FIG. 3) carried by a screen frame D composed of a pair of sections 11 extending transversely of the belt A and a pair of sections 12 extending longitudinally of the belt A, all of which are arranged in a rectangular shape, and a carriage structure G for transporting the ink container E in the lengthwise direction of the machine proper.

The bucket-like ink container E comprises a generally cylindrical barrel portion 13 having a funnelshaped bottom 14 with a circular depending flange around the central opening to define an opening 15. An annular flange 16 is integrally joined to the lowermost end of the barrel portion 13 and having a length greater than the height from the bottom edge of the opening to the upper peripheral edge of the bottom 14. A handle portion 17 is provided at the top of the barrel portion 13.

The carriage structure G for transporting the ink container E in the lengthwise direction of the machine proper to bring the container E to a printing position where the screen frame D is positioned comprises a pair of vertically spaced parallel guide rails 18a and 18b rigidly secured to one side panel 18 of the machine proper, a pair of substantially horizontally elongated chassis members 19a and 19b, each having a pair of grooved rollers 20a and 20b rotatably on the ends thereof respectively the upper chassis 19a being adapted to be guided along the upper guide rail 18a by the pair of the grooved rollers 20a while the lower chassis 19b is adapted to be guided along the lower guide rail 18b by the pair of the grooved rollers 20a. A vertically upstanding supporter stud 21 has the lower end portion and the intermediate portion rigidly connected respectively with the chassis 19b and 19a, and a swing table 22 is pivotally connected with the upper free end of the supporter stud 21 substantially at right angles thereto by means of a suitable bearing 23. This swing table 22 is rotatable about the axis of the supporter stud 21 through 360, alternatively, between two angularly spaced positions, in one of which the flanged opening 15 of the container E, when it is mounted on this swing table 22, is positioned above the screen 10 and the other in which the flanged opening 15 is positioned laterally beyond the side panel 18 of the machine proper, as shown in FIG. 4. Connected between the bearing 23 and the swing table 22 is a substantially triangular reinforcement member 24 for preventing the table 22 from bending downwardly, which might occur when an excessively heavy load is imposed on the upper surface of said table 22.

As shown in FIG. 4, the swing table has two openings 25 and 26, one of which is positioned substantially in register with the flanged opening 15 of the container E when the container is mounted on the table 22 and the other of which is positioned between the first mentioned opening 25 and a point on the peripheral edge of a circular area of the table 22 occupied by the container E. This table 22 also has on its upper surface two or more projections 22a engageable with portions of the inner peripheral surface of the annular flange 16 of the container E for preventing the latter from being accidentally displaced from a predetermined position on the table 22.

Referring particularly to FIGS. 3 and 4, the screen frame D includes, in addition to the screen 10, a pair of squeegees 27a and 27b of any known construction. However, in the instance as shown, each of the squeegees 27a and 27b comprises a rubber blade, 28a and 28b respectively, and a blade holder, 29a and 29b respectively, having a lower end connected with the rubber blade in any known manner and the other end preferably outwardly bent with respect to the corresponding end of the opposite blade holder. As is well known to those skilled in the art, these squeegees 27a and 27b are alternately reciprocated in such a manner that, during one cycle of printing which begins when the screen frame D is downwardly shifted and ends when the frame D is upwardly shifted, one of the squeegees which is positioned rearwardly of the other with respect to the direction of movement of the squeegees during the one cycle of printing is moved from one position to another to deposit the printing ink on the cloth 8 on the belt A through the screen 10 while the other squeegees is raised clear of the screen and, during the following cycle of printing, said other squeegee is moved from said another position to the initial position to deposit the printing ink on the cloth B on the belt A through the screen 10 while said one squeegee is raised clear of the screen. Of course, during each cycle of printing, the belt A is standing still relative to the printing means as hereinbefore described and, after each period of standing still the belt A with the cloth B sticking to it is moved a distance to transfer a printed area of the cloth from one position to the next position.

A traversing movement of the squeegees 27a and 27b in one direction is normally effected at one position each time the screen frame D is downwardly shifted.

For convenience of description, it is assumed that the two positions between which a pair of the squeegees 27a and 27b travels during a reciprocating traversing movement are referred to as an initial position" in which said pair of the squeegees is positioned adjacent to the automated printing ink supply device or the supporter stud 2i and a terminating position which is remote therefrom. FIGS. 3 and 4 illustrate the squeegees 27a and 27b in their initial position, i.e., at the completion of their travel from the terminating position (not shown) remote from the supporter stud 211 to the initial position adjacent to the supporter stud 21.

While in the arrangement as hereinbefore described, a proper amount of printing ink which is usually composed of an adhesive agent, oil, an emulsion, pigment, dye stuffs, catalyst and other additives and which has a viscosity of from 4,000 to 90,000 c.p.s. is normally contained in an area defined by one of the squeegees 27a, one of the longitudinally extending sections 12 of the screen frame D which is adjacent to the supporter stud 21 and the screen. As is well known to those skilled in the art, the amount of the printing ink contained in that area has a tendency to decrease little by little as the squeegees 27a and 27b which are alternately brought into the operative position as hereinbefore described are reciprocated between the initial and terminating positions substantially across the width of the machine proper, since the printing ink is deposited through the screen onto the flat surface of the cloth B lying on the endless belt A.

Accordingly, the automated printing ink supply device is herein provided for supplying a proper amount of the printing ink onto the above mentioned area to compensate for the ink being used up. Although the supply of the printing ink may be directed to any point within said area, namely, the flanged opening 15 of the container E may be positioned at any position above said area, it is preferable to direct the opening 15 at the central point of a portion of the area which is defined by the pair of the squeegees 27a and 27b and the screen 10 substantially as shown in FIG. 4. In other words, the swing table 22 is preferably rotatable about the axis of the supporter stud 21 from a first position in which the flanged opening 15 of the container E when mounted on the table 22 is in register with the central point substantially equally spaced from the ends of the squeegees 27a and 27b and midway between the pair of squeegees, to a second position in which the container E mounted on the table 22 is in a position clear of the machine proper.

The area defined by one of the squeegees 27a, one of the longitudinally extending sections 12 of the screen frame D which is adjacent to the supporter stud 2i and the screen 10 when said squeegees 27a and 27b are in the initial position and into which the printing ink is supplied as hereinbefore described is hereinafter referred to as the ink receiving area.

The carriage structure G may be provided with a locking or braking device for stopping the carriage structure G at any desired position with respect to the machine proper. For example, a threaded bolt-like stop may be utilized for this locking or braking device. In this case, the threaded bolt-like stop may have a handle on one end and the other end extending through the supporter stud 21 and terminating at the side panel of the machine proper so that, when the handle is turned until the other end of said stopper contacts the side panel of the machine proper, the carriage structure G can be fixed at one position. However, since this locking or braking device may be of any known structure and can be easily provided by those skilled in the art, the details thereof are herein omitted and not shown.

The solenoid operated valve assembly F operable to supply the printing ink to the ink receiving area on the screen 10 only when the amount or surface level of the printing ink in the ink receiving area decreases to the value lower than a predetermined value includes a slider valve 30 having a construction which will be described later and which is tightly connected to the flanged opening of the container E, an electromagnetic device 31 rigidly secured to the undersurface of the swing table 22 for operating the slider valve 30, a limit switch 32 operable in response to the vertical reciprocation of either of the squeegees 27a and 27b, and a liquid level detector 33 capable of detecting the position of a liquid surface of the printing ink contained in the ink receiving area, which may be conveniently interpreted as indicating the volume of the printing ink, and concurrently feeding an electrical signal representative of the position of the ink surface to the electromagnetic device 31, said electromagnetic device 31 being adapted to be operated so as to open the slider valve 30 according to the logical product of the electrical signal fed from the limit switch 32 and that from the liquid level detector 33.

The details of this solenoid operated valve assembly F are clearly shown in FIGS. 5 through 9. However, to begin with, the structure of the slider valve 30 will be hereinafter fully described particularly with refce to FIGS. 8 and 9.

The slider valve 30 as shown comprises a framework 30a of substantially rectangular shape having a pair of long sections and a pair of short sections, a lower plate member 30b of the same width as that of the framework 30a and having substantially at the central portion thereof an opening 300, said lower plate member 3012 being rigidly mounted on an intermediate portion of the framework 30a. An upper mounting plate member 30d having the same size as the lower plate member 30b and having an opening 30e in alignment with the opening 300 of the lower plate member 30b is mounted on framework 30a above lower plate member 30b and an annular flange 30f is provided on the upper surface of said mounting plate member 30d and surrounding said opening 302. A pair of legs (not visible) each having substantially the same thickness as that of the corresponding long section of the framework 30a is rigidly connected with said respective long sections of the framework 30a between the upper and lower mounting plate members so that a clearance is formed between the under surface of said upper mounting plate member 30d and the upper surface of the lower plate member 30b. An intermediate slidable plate member 30h is slidably accommodated in said clearance and has an opening 301' therein of the same diameter as that of the opening 300 of the lower plate member 30b, and an adjustment screw 30j is adjustably threaded through a central portion of the first one of the short sections of the framework 30a for adjusting the positioning of the intermediate slidable plate member 30h as will be described later.

The intermediate slidable plate member 30h has one end downwardly bent to form a lever 30k and the other end connected with the other short section of the framework 30a by means of a tension spring 30m adapted to pull said slidable plate member 30h in the direction away from the adjustment screw 30j. In other words, while the openings 30c and 30a are in alignment with each other, the opening 301' of the slidable plate 

1. An automated printing ink supply device for an automatic screen printing machine of the type including an endless belt adapted to be intermittently driven around a pair of drums for feeding material to be printed with the material adhered to the outer surface of said endless belt along a forwarding moving run thereof, at least one screen frame having a screen positioned above said endless belt, said screen frame being movable between an upwardly shifted position in which said screen is clear of said belt and a downwardly shifted position in which said screen is in conTact with the material on the belt during the time the belt is stopped between intermittent movements, and a pair of juxtaposed squeegees reciprocally movable between first and second positions substantially across the width of the belt and slidable over the screen to deposit a printing ink contained on said screen between said squeegees, onto the material to be printed through said screen as said pair of squeegees is moved from either of said first and second positions to the other during the time the belt is stopped, said automated printing ink supply device comprising an ink storage container having an outflow opening and adapted to contain therein a substantial amount of printing ink, container mounting means positioned at one side of the machine on which said container is mounted at a position above the screen frame and squeegees and between the position of the squeegee remote from said mounting means when said squeegees are in the first position and said screen frame at said one side of said machine, a valve connected with said outflow opening of said container for regulating the flow of the printing ink from said container onto said screen, squeegee position and frame movement responsive means actuated in response to the presence of one of said squeegees adjacent to said mounting means upon return of said pair of said squeegees to said first position and further to the raising of said frame, means for detecting the level of the surface of said printing ink on said screen and producing a response when said level falls below a predetermined level, and valve operating means connected to said valve for opening and closing it and to which said squeegee position and frame movement responsive means and said level detecting means are coupled and operable to open said valve only when said responsive means is actuated and said level detecting means produces a response whereby said valve is opened only when the level of the surface of the printing ink on the screen falls below a predetermined level when the squeegees are at the said first position.
 2. An automatic printing ink supply device according to claim 1, wherein said detecting means comprises a ultrasonic liquid level detector.
 3. An automatic printing ink supply device according to claim 1, wherein said detecting means comprises an electrode type liquid level detector.
 4. An automatic printing ink supply device according to claim 1, wherein said detecting means comprises a float type liquid level detector including a float adapted to float on the surface of said printing ink on said screen, and a limit switch adapted to be closed by said float only when said float descends beyond a predetermined position as the printing ink contained in said ink receiving area is consumed.
 5. An automatic printing ink supply device according to claim 1, wherein said opening of said valve is oriented toward the space between the pair of said squeegees.
 6. An automatic printing ink supply device according to claim 1, wherein said mounting means is a carriage mounted on the side of said machine for movement therealong in the direction of the length of said belt. 